This invention relates to methods and compositions for preventing oxidative corrosion of metals by aqueous solutions. In particular, this invention relates to methods of inhibiting oxidative corrosion in recirculating cooling water systems, and to corrosion inhibiting compositions which are useful in such systems.
Cooling water systems are widely used in oil refineries and in chemical plants, as well as in homes, factories, and public buildings. Each day huge volumes of water are being circulated through tremendous numbers of such systems. This obviously represents a large dollar volume in capital investment and operating expense. It also requires large amounts of cooling water.
Cooling water systems may be classified generally into two types. One type is the once-through cooling system, where cooling water is picked up from a convenient source, such as a river, sent once through the cooling equipment, and then discharged. Corrosion problems in such systems are generally minor. However, in most localities, cooling water is not sufficiently abundant to permit the use of a once-through system, and the number of such systems is on the decrease.
The other general type of cooling water system is the recirculating cooling water system. Recirculating systems include a cooling tower or equivalent type of equipment, where heated water from the system's heat exchanger is contacted with air from the atmosphere. During the course of such contact with air, a substantial amount of air dissolves in the cooling water and is circulated throughout the cooling system. The oxygen dissolved in the water diffuses to the water metal interface and will produce corrosion in the heat exchangers and on the metal pipes and vessels in the cooling system. Admiralty metal, copper and steel, particularly carbon steels, are the most commonly used materials in such systems, and unfortunately such materials are particularly prone to oxidative attack.
Another problem encountered in recirculating cooling water systems is the formation of scale on metal surfaces. This is due primarly to the precipitation of calcium salts and especially calcium carbonate. Iron oxides and hydroxides, formed by oxidation of iron metal in the system, can also contribute to scale formation. Even soft waters can cause some scale formation from calcium compounds, since the concentration of calcium compounds in the cooling water is several times as great as their concentration in the inlet water due to the evaporation of water in the cooling tower of the system.
The prior art has attempted to inhibit oxidative corrosion in water cooling systems by introducing various inorganic inhibitor systems which produce thin metal oxide films on the metal surfaces of the cooling systems so as to retard or hopefully prevent the diffusion of oxygen to the metal surfaces. Systems which have achieved wide acceptance in the art for this purpose include chromate and phosphate salts. Unfortunately, these systems have serious drawbacks when used as corrosion inhibitors.
Chromates under certain conditions can give rise to accelerated corrosion. For example, chromates can promote pitting when introduced in low concentrations. This pitting attack may be quite serious and may result in perforation, particularly in areas of breaks or discontinuities in the film produced by the chromate inhibitor. Since setting up virtually perfect thin film in large scale equipment with high flow conditions is tricky to say the least, it is safe to say that effective inhibition will be most unpredictable from unit to unit, and even from day to day in the same unit.
A further and most serious drawback in the use of chromates as inhibitors arises from the fact that chromates are pollutants. Chromates have toxic properties and their presence in streams and rivers is coming under even stricter control in new anti-pollution laws. Thus, in order to be able to circulate used cooling water with an environmental sewage system, it would be necessary for the cooling system operator to install adequate purification equipment to remove the chromate prior to water disposal. As a practical matter, it is very difficult and prohibitively expensive to remove chromate to an adequately low level, with the result that chromate is rapidly falling into disuse as a corrosion inhibitor.
Polyphosphates have also been used as corrosion inhibitors but with little success. These substances at very low concentrations act as sequestering agents for calcium and magnesium compounds which are present in the system. In this regard, polyphosphates are useful in water treatment. Polyphosphates in higher concentrations, e.g., about 50-100 ppm, have some beneficial effect on corrosion inhibition. However, the use of polyphosphates alone as corrosion inhibitors is limited by their rapid reversion to orthophosphates, which react with calcium ions, thus both depleting the polyphosphate concentration with an attendant increase in corrosion, and creating scale of insoluble calcium orthophosphate which seriously interferes with heat transfer. This reversion to orthophosphate increases rapidly with increasing polyphosphate concentration, decreasing pH, increasing calcium content in water, and increasing temperature. Thus, although the addition of acid to reduce the pH of cooling water decreases the amount of calcium carbonate scale formation, this advantage is at least partially offset by an increase in the formation of calcium orthophosphate. Another disadvantage of polyphosphates where used alone is that they are corrosive in concentrated solutions. In addition, polyphosphates are also stream pollutants when discharged into a sewage system, although the acceptable concentration of phosphates is considerably higher than the acceptable concentration of chromates.
Tannins and their derivatives have also been used as corrosion inhibitors as disclosed in U.S. Pat. No. 3,256,203. Unfortunately, these tannin materials have serious drawbacks when used in cooling water systems containing chlorine to retard bacterial growths since they react with the chlorine to render it ineffective as a bacteriostat.
There exists, therefore, a need for a new and effective corrosion inhibitor system which will effectively inhibit corrosion of metal surfaces in cooling water systems containing chlorine while at the same time not result in excessive concentrations of pollutants which cannot be discharged into environmental sewage systems. It has previously been proposed to inhibit the corrosion of metal surfaces in cooling water systems by adding to the water an alkali metal or ammonium gluconate. Sodium gluconate has been particularly suggested for this purpose. The gluconate salts are nontoxic in the concentrations utilized and pose no pollution problems if discharged into an environmental waste water system. Gluconates have been found to be effective for preventing corrosion. Gluconates are generally effective at concentrations of about 100 ppm or higher; however, they lose their effectiveness rapidly at concentrations below about 100 ppm.